Abstract
External coincidence counting was used for simultaneous estimation, in the 2-fluoro-2-deoxy-D-glucose (2-FDG) model, of the lumped and apparent rate constants (i.e., for a white/gray matter mixture). Eleven normal cats were studied. During a programmed 2-[18F]FDG infusion that kept a constant arterial plasma concentration of 2-[18F]FDG [Cp*(t)] for 45 minutes, the time course of cerebral tissue activity [Ci*(t)] was monitored by external coincidence counting. The apparent rate constants were estimated from Ci*(t) and Cp*(t) by a nonlinear least squares optimization method. The lumped constant (LC) estimate was obtained by fitting the ratio of extraction fractions of glucose and 2-[18F] FDG by use of a nonweighted, nonlinear least squares fitting method.
The estimated apparent rate constants, k1*, k2*, k3*, and the LC were 0.074 ± 0.005 min-1, 0.129 ± 0.007 min-1, 0.021 ± 0.001 min-1, and 0.395 ± 0.016, respectively (mean ± SEM). The focal cerebral metabolic rate for glucose detected in the brain was 28.7 ± 1.5 μmol/100 gm/min (mean ± SEM). The influence of arterial plasma glucose (CP), the LC, and the metabolic index [MI = k1*k3*/(k2* + k3*)] on the stability of the cerebral glucose utilization rate was examined. Negative correlations between CP and LC, and CP and MI were observed. However, there was a positive correlation between the LC and the MI. The method applied in this study should be very useful in the detection of changes in the kinetics of 2-[18F]FDG under pathophysiological conditions in animals. Because of the short half-life of this isotope, repeated measurements are possible.
